Great answers - I'm going to try them out and try to report back.
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S. Robert JamesMay 31 '12 at 19:44

I can tell you why a heavier weight fixes this - if you are in any stable standing or squatting position, your center of gravity must be directly above a spot that is somewhere in the rectangle formed by the heels and toes of your left and right foot. At light squat weights your center of gravity is influenced by your butt and hips. However this changes as you add more and more weight to your bar: the center of gravity is dominated by the weight on your shoulders, and any forward leaning becomes less and less possible because it would tip you forward.
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J. Won.Jun 6 '12 at 18:53

2 Answers
2

Mobility & Chest UP!

Without a form check video--which is really the gold standard, putting this text-based feedback to shame--this sounds like a mobility issue combined with not keeping your chest up.

Your back and hips (and ankles, from the sound of your other question) are not flexible enough to get into a deep squat position while maintaining a chest-up position. This is worst in the first few light sets because heavier loads on the bar push your hips deeper, helping you achieve closer to maximum mobility in the hip joint.

Keep squatting, deloading if necessary, and keep your mind blazingly focused on CHEST UP. That means squeezing the shoulderblades together, puffing out your chest like you're on Muscle Beach, and taking (and keeping) a big breath held through the whole rep. If you can't keep these up during all your sets, deload.

If the problem persists, consider switching to high-bar back squats or front squats.